Electronic Support Measures/Direction Finder sensors receives signal from emitters, creates emitter description and determine bearing to emitters. An example of such a receiver is shown in FIG. 1.
The illustrated system includes a number of antennas 12 a-c, each with an individual receiver channel 13 a-c. In the processing unit 14, the relationships between the signals received on the antennas are used to find the direction to the emitter source 11. The “signature” of the signals from an emitter 11 may be used to identify the emitter.
In order to identify an emitter, the received signals is made subject of a rather complicated processing involving the following steps:                Pulse processing, determination of (at least):                    Carrier frequency (RF)            Pulse Width (PW)            Pulse Power (P)            Time Of Arrival (TOA)            Direction Of Arrival (DOA).                        De-interleaving, (sorting of pulses by emitter) by using one or more of the calculated pulse parameters.                    The result of the de-interleaving process is a number of pulse trains, one per apparent emitter.                        Emitter Processing, determination of improved emitter describing parameters; this is mainly an improved pulse processing process using information gained in the de-interleaving process:                    Improved pulse parameters and statistics: RF, P, DOA            PRI (mean, stagger pattern, jitter pattern) by means of TOA            Emitter antenna parameters (dwell time, and scan time, rotating or oscillating) by means of P and TOA.                        
Processing of signals in ESM/DF systems are described in further detail in international patent application PCT/NO2004/000412, also owned by the present applicant.
The type of sensors described above is well known. When two or more sensors observe the same emitter, it is possible to determine the position of the emitter. With only one emitter, though, only bearing determination has traditionally been possible.
Current solutions for determining emitter position involve two or more sensors which need to communicate in order to create a cross-bearing or similar. In some operative scenarios involving mobile sensors, only one sensor is observing the emitter or radio-communication restrictions limits the ability to communicate with other sensors. Thus, with traditional methods, only bearing to the emitter is possible.
If the range, i.e. the distance to the emitter, could be determined in addition to the bearing, the position of the emitter would be known. A well known method is to use knowledge of the emitter to estimate range from received power:
  S  =                              P          t                ⁢                  G          t                ⁢                              A            r                                4            ⁢            π            ⁢                                                  ⁢                          R              2                                          ⇔      R        =                                        P            t                    ⁢                      G            t                    ⁢                      A            r                                    4          ⁢                                          ⁢          π          ⁢                                          ⁢          S                    
Where S is received signal strength, Ar is the antenna aperture, R is range. The estimate requires good knowledge of the emitter output power and antenna gain (PtGt) which must be known upfront and stored in an emitter database and also a good calibration of the ESM receiver. A minor variation of 3 dB will result in an error in the range estimate 30%, thus the accuracy of this method is usually limited. Thus, the accuracy is low at best and the method requires very good knowledge of the emitter in question.
In addition we have the effect of reflexes, which is a well known problem. ESM/DF sensors in a coastal scenario will often receive reflexes from surrounding steep cliffs, as illustrated in FIG. 2. Here, an emitter 21 is emitting signals that are scattered in several scattering points 26, 27, 28, 29 along a coastline 25 with steep cliffs. This effect may produce a multitude of apparent emitters even if only one real emitter is present. The extra emitters arising from reflected signals are called false emitters in the following description.